Light weight shaped opaque aluminum borate product and method of making same

ABSTRACT

A high strength low density opaque shaped aluminum borate product is disclosed characterized by an MOR of at least 45,000 psi and a density of approximately 2.9 g/cm 3 . The shaped aluminum borate may be formed by reacting an aluminum oxide with a boron oxide at a temperature of at least 800° C., grinding the reaction product, pressing the resulting particulate into a shaped form and sintering the shaped particulate at a temperature of from 800° to 1400° C. while maintaining the shaped particulate under a pressure of 2500 to 3500 psi.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Ray U.S. patentapplication Ser. Number 604,913 filed Apr. 27, 1984, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a light weight shaped opaque aluminum borateproduct. More particularly, this invention relates to a high strength,light weight, shaped opaque aluminum borate product formed by reactingan aluminum oxide and a boron oxide and a method of making the aluminumborate material.

2. Description of the Related Art

Lightweight ceramic materials formed from aluminum oxide mixed withphases of other metal oxides are desirable from the standpoint of weightas well as chemical inertness. In Ray U.S. patent application Ser. No.604,913, there is described and claimed a ceramic which comprises aninterwoven mixture of TiB₂ and Al₂ O₃ formed by reacting together amixture of TiO₂, B₂ O₃ and aluminum metal. This ceramic material hasbeen found to possess excellent electrical conductivity and chemicalinertness properties even at elevated temperatures, making it anexcellent material for use as an electrode in an electrolytic reductioncell at elevated temperatures.

However, such ceramic materials or cermets are not necessarilycharacterized by light weight nor do they always possess high strengthqualities, particularly if they have been blended to optimize otherproperties such as chemical inertness and electrical conductivity.

The formulation of ceramic materials from oxides of aluminum and boronwould be expected to be somewhat lighter than aluminum oxide, dependingupon the amount of boron oxide used, since the density of aluminum oxideis about 3.9 and the density of boron oxide is about 2.46. Ceramics madefrom such oxides are known.

Sowman U.S. Pat. No. 3,795,524 describes the formation of transparentextrusions such as fibers of aluminum borate and aluminum borosilicatematerials from an aqueous solution or dispersion, e.g., an aqueoussolution of a boric acid-stabilized aluminum acetate, which isconcentrated into extrudable gels, subsequently dried, and then fired attemperatures up to 1000° C. to form fibers of transparent aluminumborate or aluminum borosilicate. The patentee states that low densityaluminum borate fibers may be formed in this manner having an Al₂ O₃ :B₂O₃ mole ratio of from 9:2 to 3:1.5. Sowman, however, cautions againstfiring at temperatures as high as 1200° C, stating that fibers fired atthis temperature are weak and fragile.

DeAngelis U.S. Pat. No. 4,540,475 discloses the formation of a multiplephase body containing phases of TiB₂, Al₂ O₃ and 9Al₂ O₃ 2B₂ O₃ whichwas formed from a dry mixture of AlB₂, TiO₂ and Al₂ O3 which was pressedat 1500 psi and then fired at 1500° C.

Baumann and Moore in an article entitled "Electric FurnaceBoroaluminate" in The Journal of the American Ceramic Society, Oct. 1,1942, Vol. 25, No. 14, disclose that boroaluminate has been produced asa crystalline material by electric furnace fusion. The crystal form isorthorhombic, and it appears to melt incongruently and is analogous inseveral ways to mullite.

SUMMARY OF THE INVENTION

It has now been discovered that a moderately high strength, low density,shaped opaque aluminum borate ceramic product may be formed.

It is, therefore, an object of this invention to provide a highstrength, low density shaped opaque aluminum borate ceramic product.

It is another object of this invention to provide a high strength, lowdensity shaped opaque aluminum borate product having an MOR of at least42,000 psi and a surprisingly low density of about 2.9 grams/cm³.

It is yet another object of this invention to provide a high strength,low density shaped opaque aluminum borate product formed by reactingtogether a mixture of an aluminum oxide and a boron oxide, for example.

It is a further object of this invention to provide a method for makinga high strength, low density shaped opaque aluinum borate ceramicproduct which comprises reacting together a mixture preferably of analuminum oxide and boron oxide at a temperature of from 900° to 1400°C., particularizing the reacted mixture, and then sintering theparticulate at a temperature of from about 1300° to 1400° C. whilemaintaining a pressure of 2500 to 3500 psi on the particulate during thesintering.

The aluminum borate ceramic product formed can have a compositionsubstantially Al_(x) B_(y) O_(z) wherein x is in the range of 16 to 22,y is in the range of 2 to 5 and z is in the range of 30 to 36, and thecomposition can be formed by reacting an aluminum compound and a boroncompound, at least one of the compounds containing oxygen.

These and other objects of the invention will be apparent from thefollowing description and accompanying flow sheet.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow sheet illustrating the process of the invention.

FIG. 2a, 2b, 2c, 2d, and 2e are a series of micrographs showing theproduct formation in accordance with the invention at differenttemperatures.

DETAILED DESCRIPTION OF THE INVENTION

The high strength and low density shaped opaque aluminum borate ceramicproduct of the invention may be characterized by the formula Al_(x)B_(y) O_(z) wherein x=16 to 22, y=2 to 5 and z=30 to 36. Preferably, thehigh strength and low density aluminum borate material of the inventioncomprises, in one aspect of the invention, stoichiometry represented bythe formula Al₁₈ B₄ O₃₃.

The high strength, low density aluminum borate may be formed byinitially mixing together a powder mixture of finely ground aluminumoxide and boron oxide. The aluminum oxide powder may comprise Alcoa A-16super ground alumina having an average particle size of 0.4 micrometerwhile the boron oxide may comprise Fisher A-76 B₂ O₃ powder or boriapowder which should have an average particle size of about -325 mesh(Tyler). The powder mixture may then be heated at from 900 to 1400° C.for a period of from 5 minutes to 2 days to form a fiber product, forexample. Optionally, the powder mixture may be first pressed into agreen ceramic and then heated.

Other aluminum compounds, e.g., Al₂ O₃.3H₂ O, Al₂ O₃.H₂ O, AlCl₃.6H₂ O,AlCl₃, and Al(NO₃)₃.9H₂ O, may be used along with boric acid, e.g., H₃BO₃, to produce the material, e.g. powder or fibers, for formng into theceramic product.

In either embodiment, the heated material may be particularized such asby grinding or the like to a powder size of -325 mesh (Tyler), e.g., 0.4to 30 micrometers, and typically an average particle size of 0.6 to 1.0micrometer and then pressed into the desired shape and sintered at atemperature of from about 1300° to 1400° C. for a period of from about 5minutes to 2 hours while maintaining the ceramic under a pressure ofabout 2500 to 3500 psi using a non-reactive die surface such as agraphite surface.

The resulting shaped aluminum borate product comprises a high strength,low density gray opaque material having an MOR of approximately 47,000psi, a hardness of 1300 VPN and a density of 2.92 grams per cubiccentimeter (g/cm³).

While the inventor does not wish to be bound by any theory of why theresulting aluminum borate material possesses such strength whileexhibiting such a low density, it appears that the aluminum oxide andboron oxide are reacting together to form a crystal structure lessdense, i.e., more open, then that of the aluminum oxide. Thus eventhough the less dense boron oxide material is only present in analuminum oxide/boron oxide molar ratio of 9:2, the reduction of thedensity of the aluminum borate product from that of aluminum oxide is,surprisingly, a drop of from 3.9 down to 2.9. That is, since the molarpercentage of the aluminum oxide is 82% (87 wt. %) in the aluminumborate composition and that of the boron oxide only 22% (13 wt. %), onewould not expect such a drastic reduction in density in the finalproduct from that of the predominantly aluminum oxide component absent achange in the crystallography of the resultant product. Furthermore, itmust be noted that the resulting product, apparently due to the pressureused in forming the product, has a density which is almost 99% oftheoretical, i.e., there is very little porosity in the final product.

Aluminum borate materials formed in accordance with the invention may beused as wear and abrasive surfaces for dies for drawing, forming, orextrusion; finishing operations such as cutting tools and machiningtools; surface coatings such as, for example, marble-like floorcoatings: adiabatic engine components; and protective cladding. Thealuminum borate materials of the invention may also find utility incombination with other materials such as dispersion strengthened metalsand structural composites of metal and ceramic matrices.

The aluminum borate material of the invention may also be blended withup to 95 vol. %, typically 25 to 50 vol. % of other materials to obtaincomposites with other properties. For example, the aluminum borate maybe mixed with TiB₂ or ZrB₂ to obtain a composite product, if desired,while retaining the high strength properties of the aluminum borate. Al₂O₃ and other metal oxides, borides, carbides and/or nitrides may also beblended with the aluminum borate. To provide a material with enhancedtoughness, from 5 to 30 vol. % of a mixture of ZrO₂ --Y₂ O₃ or HfO₂ --Y₂O₃ may be added to the aluminum borate. Preferably such materials areadded to the powdered mixture of aluminum borate prior to thepressurized sintering step.

The pressure may be dispensed with during the sintering step if it isdesired to produce a more porous product. For example, an aluminumborate having a porosity of 20% may be produced by sintering theparticulate in an open furnace without restraining the sintered mass. Aporous aluminum borate and titanium or zirconium boride mixture may alsobe obtained by reaction sintering a mixture of titanium or zirconiumoxide with boria, alumina and aluminum in accordance with one of thefollowing formulas:

    3XO.sub.2 +9B.sub.2 O.sub.3 +10Al+22Al.sub.2 O.sub.3 →3XB.sub.2 +3Al.sub.18 B40.sub.33

    3XO.sub.2 +5B.sub.2 O.sub.3 +10Al+4Al.sub.2 O.sub.3 →3XB.sub.2 +Al.sub.18 B40.sub.33

Wherein X=titanium or zirconium

Such a porous material could be further modified by impregnating thepores of the ceramic material with a metal such as aluminum or steel toproduce a cermet with enhanced properties.

A mixture of powder containing 86.83 wt. % alumina A-16-SG and 13.17 wt.% boron oxide was mixed and calcined at 800° C. The materials resultingfrom the calcination are shown in FIG. 2a. This material was then heatedfurther to 900° C. and is shown in FIG. 2b. It will be noted that aftertreating to 900° C., some of the particles are needle shaped. The samewas true when the material was further treated to 1000° C.

On heating up to 1000° C., it will be noted that the major constituentsare Al₂ O₃ and 2Al₂ O₃.B₂ O₃ type phase. On further heating to 1150° C.,that there was a major phase change: that is, the major constituent formwas Al₁₈ B₄ O₃₃ and the minor was Al₂ O₃ (FIG. 2d). The material wasfurther heated to 1300° C. and is shown in FIG. 2e. It will be seen thatcalcination of alumina and boria powder at a temperature of about 1150°C. resulted in the formation of Al₁₈ B₄ O₃₃.

The Al₁₈ B₄ O₃₃ material was ground to a particle size in the range of0.5 to 10 micrometers and subsequently hot pressed at 1350° C. in agraphite mold to provide a density of 2.92 gm/cc (99% dense). Theceramic material formed had a thermal expansion coefficient (3.9×10⁻⁶cm/cm/° K.), and thermal conductivity of 0.065 W/cm/K. In addition, thematerial had a MOR strength of 47,000 psi, a hardness of 1300 VPN and anelastic modulus of 31.8×10⁶ psi.

Thus the shaped aluminum borate product of the invention comprises ahigh strength and low density material which may be produced in avariety of formed shapes. The product may be used by itself or as avaluable precursor for use in connection with other materials to achievea variety of desired physical properties.

Having thus described the invention, what is claimed is:
 1. A shapedopaque high strength low density aluminum borate product produced byforming a powder mixture of aluminum oxide powder and boron oxidepowder, said powder having a particle size of -325 mesh (Tyler);reacting said powder mixture for a period of from 5 minutes to 48 hoursat a temperature of from about 800° to about 1400° C. to form aluminumborate grinding said aluminum borate reaction product to a particulatehaving an average particle size of -325 mesh (Tyler); pressing saidparticulate into a shaped object: and sintering said shaped object for aperiod of from 5 minutes to 2 hours at a temperature of from 800° to1400° C. while maintaining said shaped object under a pressure of fromabout 2500 to 3500 psi to form said shaped aluminum borate product.
 2. Ashaped high strength sintered aluminum borate product in accordance withclaim 1 characterized by an MOR of at least 45,000 psi and a density ofapproximately 2.9 g/cm³.
 3. The product in accordance with claim 1wherein the aluminum compound is selected from Al₂ O₃.3H₂ O, Al₂ O₃.H₂O, AlCl₃.6H₂ O, AlCl₃ and Al(NO₃)₃.9H₂ O.
 4. The product in accordancewith claim 1 wherein the aluminum compound is aluminum oxide and theboron compound is boron oxide.
 5. A shaped high strength sinteredaluminum borate ceramic product consisting essentially of 70 to 95 vol.% of Al_(x) B_(y) O_(z) wherein x is in the range of 16 to 22, y is inthe range of 2 to 5, and z is in the range of 30 to 36, and from 5 to 30vol. % of a material added prior to a sintered step to provide enhancedtoughness to said sintered ceramic product and selected from the classconsisting of ZrO₂ --Y₂ O₃ and HfO₂ --Y₂ O₃ ; said shaped high stengthsintered aluminum borate ceramic product produced by forming an initialaluminum borate material from an aluminum compound and a boron compound,wherein at least one of said compounds contains oxygen; particularizingsaid aluminum borate material to a particulate having an averageparticle size of -325 mesh (Tyler); pressing said particulate into ashaped product; and sintering said shaped product into a shaped highstrength sintered aluminum borate ceramic product.
 6. The aluminumborate product of claim 2 wherein said product consists of a sinteredbody formed by sintering at a temperature of from 800° to 1400° C., aparticulate mixture having an average particle size of -325 mesh (Tyler)while maintaining the reaction mass under a pressure of 2500 to 3500psi.
 7. The shaped high stength sintered aluminum borate ceramic productof claim 5 wherein the major constituent of said initial aluminum boratematerial consists essentially of Al₁₈ B₄ O₃₃.
 8. A process for making ashaped high stength sintered aluminum borate ceramic product whichcomprises:(a) reacting an aluminum compound with a boron compound at atemperature of at least 800° C. to form an aluminum borate material, atleast one of said compounds containing oxygen; (b) particularizing saidaluminum borate material to a average particle size of -325 mesh (Tyler)or less; (c) pressing said aluminum borate particles into a shapedobject; and (d) sintering said shaped aluminum borate object at atemperature of at least about 800° C. to form said shaped high strengthsintered aluminum borate ceramic product.
 9. The process of claim 8wherein said step of reacting an aluminum compound with a boron compoundfurther comprises reacting aluminum oxide with boron oxide.
 10. Theprocess of claim 9 wherein said step of reacting aluminum oxide withboron oxide at a temperature of at least 800° C. further comprisesreacting said aluminum oxide with said boron oxide at a temperature ofat least 1150° C. to form an aluminum borate having as its majorconstituent Al₁₈ B₄ O₃₃.
 11. The process of claim 8 wherein said step ofsintering further comprises sintering said shaped product at atemperature of from about 800° to 1400° C. to form said shaped highstrength sintered aluminum borate ceramic product.
 12. The process ofclaim 8 wherein said step of sintering further comprises sintering saidshaped product at a temperature of from about 1300° to 1400° C. to formsaid shaped high strength sintered aluminum borate ceramic product. 13.The process in accordance with claim 8 wherein said aluminum compound isselected from Al₂ O₃.3H₂ O, Al₂ O₃.H₂ O, AlCl₃.6H₂ O, and AlCl₃Al(NO₃)₃.9H₂ O.
 14. The process of claim 8 wherein said reactiontemperature is from at least 800° to about 1400° C.
 15. The process ofclaim 13 wherein said aluminum compound and said boron compound eachcomprise powdered material have a particle size of -325 mesh (Tyler) orfiner.
 16. The process of claim 14 including the further step of formingsaid particles into a shaped mass prior to said reaction.
 17. Theprocess of claim 8 wherein said reaction step is carried out over aperiod of from 5 minutes to 48 hours.
 18. The process of claim 8 whereinsaid sintering step is carried out while maintaining said particulateunder a pressure of from 2500 to 3500 psi.
 19. The process of claim 9wherein said sintering step is carried out over a period of from 5minutes to 2 hours.
 20. A process for forming an opaque high strengthlow density shaped aluminum borate product which comprises the stepsof:(a) forming a powder mixture of aluminum oxide and boron oxidepowder, said powder having an average particle size of -325 mesh(Tyler); (b) reacting said powder mixture for a period of from 5 minutesto 48 hours at a temperature of from about 800° to about 1400° C. toform aluminum borate; (c) grinding said aluminum borate reaction productto a particulate having an average particle size of -325 mesh (Tyler):(d) pressing said particulate into a shaped object; and (e) sinteringsaid shaped object for a period of from 5 minutes to 2 hours at atemperature of from 800° to 1400° C. while maintaining said shapedobject under a pressure of from about 2500 to 3500 psi to form saidshaped aluminum borate product.